Diagnostic Guide: P0137 - O2 Sensor Circuit Low Voltage Bank 1 Sensor 2

What This Code Means

• P0137 is a powertrain diagnostic trouble code indicating an O2 sensor circuit problem: O2 Sensor Circuit Low Voltage for Bank 1 Sensor 2 (the downstream O2 sensor after the on the bank that contains cylinder 1). In OBD-II terminology, Bank 1 Sensor 2 is the downstream sensor used to monitor catalyst efficiency. The sensor's voltage is monitored by the PCM; a persistently low signal triggers P0137.

Symptoms

• Malfunction Indicator Lamp (MIL) illuminated or flashing.
• Decent chance of noticing reduced fuel economy or occasional roughness if other circuits are affected.
• In many cases, driveability feels normal, and the MIL is the primary symptom; some vehicles may exhibit subtle idle/low-load issues when the downstream sensor isn't providing stable data.
• Expectation that the downstream O2 sensor voltage stays low (as viewed by scan tool) regardless of engine conditions.

Notes on symptoms: Because P0137 concerns the downstream (Bank 1 Sensor 2) O2 sensor, driveability issues are not always dramatic. The more reliable symptom is the persistent low voltage indication in the downstream sensor data and the MIL being active.

Probable Causes

• Most likely: Faulty downstream O2 sensor (Bank 1 Sensor 2) or its heater circuit. 40-50%
• Wiring/connector problems (damaged, corroded, chafed, or disconnected harness/ground/signal or heater wires). 25%
• Exhaust system issues or leaks near the sensor (upstream leaks before the sensor area or around the sensor can affect readings). 10-15%
• PCM/ECU wiring or internal fault affecting the sensor signal path. 5-10%
• Sensor heater circuit issue that prevents proper warming (often correlated with a bad sensor or wiring). 5-10%

What to check (diagnostic workflow)

1) Verify the code and related data

• Confirm P0137 is current and not a stale/cleared code that reappeared after a transient condition.
• Check freeze-frame data for Bank 1 Sensor 1 (upstream) and Bank 1 Sensor 2 readings, fuel trim values, engine RPM, load, and catalyst status at the time of code setting.
• Look for other O2 codes (e.g., P0130, P0133, P0134) that might indicate upstream sensor issues or broad O2 system problems.

2) Visual inspection

• Inspect the O2 sensor 2 (downstream, Bank 1) harness and connector for signs of damage, corrosion, oil/fuel contamination, or loose connections.
• Inspect the wiring harness routing for chafed insulation, kinks, or heat damage near hot exhaust components.
• Inspect for exhaust leaks upstream of the sensor or around the sensor mount that could affect readings.

3) Compare upstream vs downstream O2 sensor data

• Using a scan tool, view live data for:
  • Bank 1 Sensor 1 (upstream) voltage and how it switches (generally toggling between ~0.1-0.9 V as the engine runs).
  • Bank 1 Sensor 2 (downstream) voltage (the subject of P0137).
• If Bank 1 Sensor 1 is switching normally and Bank 2 Sensor 2 remains statically low, the issue is most consistent with a downstream sensor problem or its circuit.

4) Check the downstream oxygen sensor heater circuit

• Measure the resistance of the heater element in Bank 1 Sensor 2 (sensor removed from the exhaust) using a multimeter per the sensor's spec (typical heater resistance on many O2 sensors is in the single-digit to low double-digit ohms range, e.g., ~5-20 Ω depending on design).
• Verify the heater supply circuit from the PCM/engine fuse and relay; inspect the heater circuit wiring for shorts to power or to ground.
• If the heater is open or significantly out of spec, the sensor may not reach its operating temperature, causing low voltage readings or lack of proper switching.

5) Inspect for exhaust/system conditions that affect downstream readings

• Look for leaks before or around the downstream sensor (including collector or pipe joints) that could introduce additional oxygen and affect readings.
• Consider a issue or other exhaust-side conditions that could render downstream readings misleading, especially if the catalyst is not functioning properly.

6) Perform a sensor health check and/or swap test

• If wiring and exhaust checks are OK, consider testing the sensor by swapping Bank 1 Sensor 2 with a known-good sensor (if you have a spare or a test harness). Re-check the code and downstream sensor voltage.
• If the problem follows the sensor, replace Bank 1 Sensor 2 and recheck.
• If the code remains after sensor replacement, re-check upstream O2 sensor (Bank 1 Sensor 1) behavior and wiring, as upstream anomalies can indirectly affect downstream readings and PCM compensation.

7) Re-check after repair and perform a drive cycle

• Clear codes and perform a steady drive cycle that includes closed-loop operation (engine at normal operating temperature, moderate load, varied throttle).
• Confirm the MIL does not reappear and that downstream O2 sensor voltage stabilizes within expected ranges and responds appropriately to any catalyst-related feed-back (though downstream sensor typically shows smaller swings than upstream).

8) If issues persist

• Re-check for intermittent wiring faults (loose connectors, corrosion, or harness damage that only shows under vibration or heat).
• Consider PCM or software updates if OEM service information indicates issues with sensor data interpretation. This is less common, but worth checking for some vehicles.

Suggested test plan (step-by-step)

• Step A: Visual inspection of Bank 1 Sensor 2 harness and connector; repair/repair as needed.
• Step B: Live data check: Bank 1 Sensor 1 vs Bank 1 Sensor 2; note if Sensor 2 voltage remains low or unresponsive.
• Step C: Heater circuit test: measure sensor heater resistance and verify heater supply voltage (5 V reference and control from PCM as applicable by vehicle).
• Step D: Exhaust leak check around sensor area; repair leaks if found.
• Step E: If feasible, swap in a known-good Bank 1 Sensor 2; recheck codes and data.
• Step F: If code persists after sensor swap, re-check upstream sensor operation; scan for related codes and verify overall catalyst status.

What you might repair or replace

• Primary remedy: Bank 1 Sensor 2 O2 sensor replacement if sensor voltage is consistently low, heater circuit is good or shows failure, and wiring is intact.
• Wiring/connector repair: fix damaged wires, restore proper ground, replace connectors or protective sleeves as needed.
• Exhaust plumbing: repair any leaks upstream of the downstream sensor.
• Catalyst-related concerns: generally not the first target for P0137 unless supported by other codes or test results.

Safety Considerations

• O2 sensors and exhaust components can be extremely hot. Allow the exhaust to cool before touching components.
• Use appropriate PPE; when disconnecting electrical connectors, ensure the battery is disconnected if recommended by the service procedure.
• Work in a well-ventilated area; avoid inhaling fumes and ensure the vehicle is secured on a level surface with stands if you lift the vehicle.
• When testing the heater circuit, be mindful of high current pathways and avoid shorting the circuit to prevent ECU damage or fusing issues.

How this relates to the sources

• Diagnostic Trouble Codes and Powertrain Codes context: OBD-II and DTCs provide a framework for understanding that P0137 is a powertrain code related to the O2 sensor circuit. This supports the diagnostic approach of checking sensors, wiring, and exhaust systems.
• Open-source definitions: The GitHub entry describing aligns with the concept that downstream O2 sensor circuits can present as low voltage (and maps to P0137 in the downstream sensor context). This supports the sensor- and circuit-level focus of the diagnostic steps.
• Emissions and testing context: The OBD-II emissions-oriented sections reinforce that downstream O2 sensors monitor catalyst efficiency and are part of the emissions control system, which is consistent with the expectation to diagnose sensor health, wiring, and exhaust leaks.

Notes

• If your vehicle's service literature (OEM procedures) provides a specific sequence for P0137 diagnostics, follow it first, as some vehicles have vendor-specific steps or test values.
• Always check for related codes (e.g., P0130, P0133, P0134) that might indicate broader O2 sensor issues or upstream sensor anomalies.
• Document freeze-frame data and any sensor readings with timestamps during diagnostic steps to track improvements after repairs.

References (context )

• Wikipedia - OBD-II: Diagnostic Trouble Codes; Powertrain Codes (context for DTCs and how OBD-II codes are organized and used). These sections help frame P0137 as a powertrain diagnostic code related to O2 sensor circuits.
• Open Source - OBD2 CODE DEFINITIONS: (describes low voltage characteristics for O2 sensor circuits in Bank 1, aligning with the downstream sensor discussion used for P0137). This provides a terminology anchor for in Bank 1.
• Note: The P0137 mapping to Bank 1 Sensor 2 is the standard interpretation in OBD-II code catalogs; downstream sensor is the focus here, consistent with the described open-source entry and general OBD-II code naming conventions.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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